Energy consumption in the United States alone was 97.7 quads in 2011 and is projected to rise to 102.3 quads by 2025 and to 107.6 quads by 2040. (1 quad≈1018 Joule.) The amount of energy derived from fossil fuels (petroleum, natural gas, and coal) is estimated to be ˜80% of total energy consumption. There is an unfilled need for alternative and renewable energy sources. Biomass is both renewable and abundant. Ample supplies of agricultural and forestry residues could potentially be converted into usable energy sources. In the United States alone the annual availability of unused wood residues from logging and thinning is estimated to be ˜97 million dry tons.
Since the 1970's, research has focused on various ways for upgrading lignocellulosic biomass into fuels and other industrially valuable chemicals. The two most common methods for converting biomass to usable energy sources are: (i) biochemical conversion, such as anaerobic digestion and fermentation, and (ii) thermochemical conversion, such as incineration, gasification, and pyrolysis. Thermochemical conversion technologies are usually preferred because they are more easily implemented into the existing energy infrastructure. Pyrolysis, a thermochemical process, has received considerable attention, not only as a precursor to combustion and gasification processes, but also as an independent process in its own right.
Pyrolysis creates high-energy products with numerous potential uses. Pyrolysis is energy-efficient, and it can be self-sustaining. However, pyrolysis is still in the early developmental stage. Further research and development are needed to make pyrolysis competitive with other renewable energy technologies.
Pyrolysis Overview
Biomass pyrolysis is the thermochemical decomposition of biomass at elevated temperatures, in the absence of significant levels of oxygen gas. As the biomass is heated it decomposes into volatile vapors, which are then rapidly condensed to form “bio-oil.” The remaining products are char and non-condensable gases. Each of these products has numerous applications. The char can be used to amend soils; it may be converted into activated carbon, or it may be used in a carbon-based catalyst. The excess non-condensable gas primarily comprises combustible gases such as H2, CO, C2H2, CH4, etc. These gases can optionally be redirected to supply energy to drive the pyrolysis process itself. Finally, the liquid bio-oil can be upgraded via for use as a hydrocarbon fuel or other industrial chemical. “Upgrading” typically implies hydrogenation or hydrodeoxygenation; but it can also include catalytic cracking to alter the relative mixture of products, as is often done when refining conventional petroleum.
Bio-oil is the most value product of pyrolysis. It results from the depolymerization and fragmentation of biomass feedstock components (e.g., cellulose, hemicellulose, and lignin) during pyrolysis. Bio-oil is a complex mixture of different sized (mostly relatively large) organic molecules such as phenols, furans, levoglucosan, and other compounds. Nearly all species of oxygenated organics are present in bio-oil, including aldehydes, ketones, alcohols, ethers, esters, phenols, carboxylic acids, etc. The molecules that compose the bio-oil liquid are generally highly oxygenated.
The yield and characterization of pyrolysis products are based on feedstock composition and reaction conditions. For example, there is a higher yield of non-condensable gases when pyrolysis is conducted at higher temperatures and longer residence times. There is higher yield of bio-oil at higher temperatures and shorter residence times; and there is more char at lower temperatures and shorter residence time.
Microwave Heating Overview
Microwave heating generates heat within materials, in contrast to conventional heating, which heats the surface of materials. Conventional methods use conduction, convection, or radiation; and the resulting surface temperature is substantially greater than the core temperature. Heat transfer from the surface of the material to the center is slow and inefficient. Microwaves are electromagnetic radiation that can generate heat via the interaction of a molecular dipole with the alternating electric field. (Microwaves are generally defined as having wavelengths between ˜1 mm and ˜1 m, corresponding to frequencies from ˜300 MHz to ˜300 GHz.) By converting the electromagnetic field into heat, the material can be heated both at its core and at its surface. Depending on the details of a particular configuration, in some cases the core temperature can even exceed the surface temperature. Microwave irradiation permits rapid, more uniform, and more selective heating. The conversion efficiency of microwave energy into heat is high, typically ˜80%-85%.
The effect of microwaves on a material depends on the dielectric properties of the material; not all materials react similarly. There are three principal ways in which material can interact with the electric part of a microwave field: (i) as an insulator that is microwave-transparent, through which microwaves pass with little loss, (ii) as a conductor that reflects microwaves and thus blocks microwaves from passing through the material, and (iii) as an absorber of microwave energy.
Microwave-Assisted Pyrolysis
The dry biomass that is typically used in pyrolysis reactions tends to be a microwave insulator, with poor absorbing properties. In practice, dry biomass typically does not absorb enough microwave energy to heat to an effective pyrolysis temperature. Therefore microwave-assisted pyrolysis usually depends on heating a microwave absorber, e.g., char, a catalyst, or activated carbon. In a mixture of biomass with a microwave absorber, microwaves are first absorbed primarily by the microwave absorber, which then conducts heat to the biomass for the latter to reach pyrolysis temperatures.
Inverted heat transfer and high temperature can be achieved with microwave-assisted pyrolysis. However, microwave-assisted pyrolysis has been slow to be commercialized due to the cost of existing techniques. There is an unfilled need for more efficient, less expensive methods of conducting microwave-assisted pyrolysis. Previous studies have primarily been conducted with a batch or semi-batch process; few have been conducted as a continuous process.
Ren, S. et al., “Biofuel Production and Kinetics Analysis for Microwave Pyrolysis of Douglas Fir Sawdust Pellet,” J. of Analytical and Applied Pyrolysis, Vol. 94, pp. 163-169 (2012) investigated the impact of reaction temperature and residence time on product yields from microwave-induced pyrolysis of Douglas fir sawdust pellets. The results showed that, in general, bio-oil and syngas yields increased with increasing temperatures and longer residence times. The chemical composition of the bio-oil and syngas were found to be highly dependent on reaction temperature.
Borges, F. C. et al., “Fast Microwave Assisted Pyrolysis of Biomass Using Microwave Absorbent,” Bioresource Tech., Vol. 156, pp. 267-274 (2014) reported fast microwave-assisted pyrolysis of biomass mixed with a microwave absorber.
Zhou, R. et al. “Effects of reaction temperature, time and particle size on switchgrass microwave pyrolysis and reaction kinetics.” International Journal of Agricultural and Biological Engineering vol. 6, pp. 53-61 (2013) investigated the effect of reaction temperature, residence time, and particle size on microwave pyrolysis of switchgrass. The authors concluded that thermochemical reactions can take place rapidly via microwave pyrolysis in materials having large particle sizes. Thus feedstock grinding may not be necessary for microwave pyrolysis.
Bu, Q. et al., “Production of Phenols and Biofuels by Catalytic Microwave Pyrolysis of Lignocellulosic Biomass,” Bioresource. Tech., Vol. 108, pp. 274-279 (2012) studied the effect of microwave absorbers on catalytic microwave pyrolysis, and concluded that adding activated carbon had a significant impact on phenols in the bio-oil product.
Lei, H., et al., “Microwave pyrolysis of distillers dried grain with solubles (DDGS) for biofuel production,” Bioresource technology, vol. 102, pp. 6208-6213 (2011) studied bio-oil production by microwave pyrolysis of distillers' dried grain. About 13 wt % of the bio-oil, without upgrading, had the same hydrocarbon composition as unleaded gasoline.
Tian, Y. et al., “Estimation of a Novel Method to Produce Bio-Oil from Sewage Sludge by Microwave Pyrolysis with the Consideration of Efficiency and Safety,” Biores. Tech., Vol. 102, pp. 2053-2061 (2011) investigated microwave absorbers for the microwave-assisted pyrolysis of sewage sludge. Different microwave absorbers (graphite, residue char, active carbon, or silicon carbide) were mixed with sewage sludge feedstocks. Each was shown to alter reaction conditions, which in turn affected product yields and characteristics.
Induction Heating Overview
Induction heating is a contactless heating method in which an alternating voltage is applied to an induction heating coil. The coil contains a conductive material arranged in a series of loops. Applying an alternating voltage creates an alternating magnetic field on the load (the loops of conductive material), which in turn produces heat via eddy currents and magnetic hysteresis. Eddy currents oppose the applied magnetic field and are the primary source of heat for induction heating. Magnetic hysteresis creates additional heat in ferromagnetic materials. The advantages of an induction heating system are rapid heating due to high power densities, high energy efficiency, and accurately controlled heating. Induction heating has potential use in bio-oil production because these characteristics can help generate high liquid yields from pyrolysis.
Tsai, W. T. et al., “Fast Pyrolysis of Rice Husk: Product Yields and Compositions,” Bioresource Tech., Vol. 98, no. 1, pp. 22-28 (2006) reported the use of induction heating for fast pyrolysis of rice husks. The resulting pyrolytic oil was a complex mixture of aromatic and carbonyl compounds. Similar experiments were performed with sugarcane bagasse and coconut shells with the same tubular reactor. The resulting liquid yields were similar, but up to 65% of the liquid yield was water, due both to high water content in the feedstock and to dehydration reactions that occurred during pyrolysis.
Other experiments have used induction heat for pyrolysis of Napier grass, a non-foraged grass. Lee, M. K. et al., “Pyrolysis of Napier Grass in an Induction-Heating Reactor,” J. of Analytical and Applied Pyrolysis, Vol. 88, no. 2, pp. 110-116 (2010) reported that high concentrations of water and oxygenated compounds in the liquid fraction meant that further processing would be needed before the produced bio-oils could be used as fuels.
Induction Upgrading
Pyrolysis bio-oil is produced by rapidly heating biomass at a very high temperature in anoxic conditions, followed by rapid quenching. The initial condensate is thermodynamically unstable. The thermodynamically unstable product tends to move towards thermodynamic equilibrium during storage, resulting in polymerization and repolymerization reactions, which increase the product's viscosity and reduce its heating value.
Other factors limiting the use of bio-oil produced by fast pyrolysis include its high oxygen content, high acidity, and high ash content. Pyrolysis bio-oil typically has an oxygen content of about 40%, compared to less than 1% in conventional hydrocarbon fuels. A high oxygen content means the energy density can be up to 50% lower, and it can also make the bio-oil immiscible with other hydrocarbon fuels. To overcome these problems, bio-oil is usually “upgraded” before the bio-oil is refined. “Upgrading” typically involves one or more of the following: removal of water, hydrodeoxygenation, thermocatalytic cracking, emulsification, and steam reforming. Existing upgrading catalysts perform poorly with bio-oils due to active site poisoning, and re-polymerization of bio-oil constituents that block the catalyst pores.
Both thermocatalytic cracking and hydrodeoxygenation reduce the oxygen content of the bio-oil. In the past, these techniques have typically required complicated and sophisticated equipment, which increases the processing costs. Catalyst deactivation and reactor clogging also add to the cost.
Another method for upgrading pyrolysis bio-oil is emulsification of the bio-oil with conventional diesel. Since bio-oil and conventional hydrocarbon fuels are immiscible, they are combined by emulsification using surfactants. Interestingly, the emulsions tend to be more stable than bio-oil alone. In addition, the viscosity of the emulsion is typically lower than that of the bio-oil alone. However, the production cost is higher, and emulsification does not solve other problems such as corrosiveness.
Catalytic Cracking Overview
Catalytic cracking is an effective way to reduce the oxygen content of bio-oil. Pyrolysis vapors from the thermochemical decomposition of biomass are passed over a hot catalyst bed. Deoxygenation reactions on the catalyst surface break higher molecular weight compounds down to lower molecular weight hydrocarbons, and oxygen is released—primarily as water, CO2, and CO.
A disadvantage of existing pyrolysis upgrading methods is that coke tends to deposit on the catalyst, leading to catalyst deactivation and non-uniform heating. Non-uniform, cooler temperature zones can lead to repolymerization reactions on catalyst surfaces, while hotspots favor higher gas yields. Conventional heating methods are not energy efficient, they have slower heating and cooling rates, and they tend to produce temperature gradients (non-uniform temperature distributions). In conventional methods, the catalyst bed is heated by a carrier such as sand, or by heat exchangers that surround the bed. The temperature is maintained by a cooler fluid that absorbs excess heat. Heat loss can be significant during energy transfer from the carrier or heat exchanger to the catalyst, and then to the coolant.
Adam, J. et al., “Pyrolysis of Biomass in the Presence of Al-MCM-41 Type Catalysts,” Fuel, Vol. 84, no. 12-13, pp. 1494-1502 (2005) reported the effect of three different catalysts with enlarged pores on bio-oil composition (Al-MCM-41, Cu/Al-MCM-41, and Al-MCM-41). The authors observed that the compositions of the resulting bio-oils differed significantly. Levoglucosan was eliminated, while furan, aromatics, and acetic acid increased.
Adjaye, J. D. et al., “Production of Hydrocarbons by Catalytic Upgrading of a Fast Pyrolysis Bio-Oil. Part II: Comparative Catalyst Performance and Reaction Pathways,” Fuel Proc. Tech., Vol. 45, no. 3, pp. 185-202 (1995) studied the effect of five catalysts: HZSM-5, H-Y, H-mordenite, silicate and silica alumina. HZSM-5 produced the highest hydrocarbon yield. The results suggested that bio-oil conversion can follow different pathways: thermal or thermocatalytic. The thermal pathway breaks down high molecular weight compounds into lighter fractions. The thermocatalytic pathway produces coke, gas, and water, and higher levels of aromatic compounds.
Aguado, R. et al., “Pyrolysis of Sawdust in a Conical Spouted Bed Reactor. Yields and Product Composition,” Industrial & Engineering Chem. Res., Vol. 39, no. 6, pp. 1925-1933 (2000) reported that in situ catalytic flash pyrolysis increased the gas and char yield and decreased the liquid yield. Also, CO2 yield decreased and C4 yield increased significantly. (“C4” generically denotes all 4-carbon hydrocarbons, including n-butane, isobutene, 1-butene, (Z)-2-butene, (E)-2-butene, butadiene, etc.)
Nguyen, T. S., et al., “Catalytic upgrading of biomass pyrolysis vapours using faujasite zeolite catalysts,” Biomass and bioenergy, vol. 48, pp. 100-110 (2013) studied the effect of a Faujasite zeolite catalyst on biomass pyrolysis. They reported that the resulting upgraded bio-oil vapors had superior fuel quality as compared to that produced by in situ catalytic upgrading. Char, water, and gas yields all increased, while liquid yield decreased. The upgraded bio-oil was richer in aromatic compounds. An Na0.2H0.8-FAU catalyst was most effective in oxygen removal. Catalyst upgrading also reduced levels of aldehydes, ketones, and acids, which increased the energy content of the oil.
Platinum's hydrodeoxygenating catalytic properties make it useful in many applications, such as the removal of trans fatty acids in cottonseed oil via hydrogenation, the electroreduction of oxygen in fuel cells, and the hydrodechlorination of tetrachloromethane.
Adjaye, J. D. et al., Production of Hydrocarbons by Catalytic Upgrading of a Fast Pyrolysis Bio-Oil. Part II: Comparative Catalyst Performance and Reaction Pathways,” Fuel Processing Tech., Vol. 45, no. 3, pp. 185-202 (1995) reported that zeolites such as HZSM-5 are among the most effective catalysts in deoxygenating bio-oil.
ZSM-5 and other zeolite catalysts have been deposited as conformal thin film coatings. For example, Louis, B. et al., Synthesis of ZSM-5 coatings on stainless steel grids and their catalytic performance for partial oxidation of benzene by N2O. Applied Catalysis A: General 2001, 210 (1-2), 103-109 described hydrothermal reactions over ZSM-5 on a stainless steel grid.
Seijger, G. B. F. et al., In situ synthesis of binderless ZSM-5 zeolitic coatings on ceramic foam supports. Microporous and Mesoporous Materials 2000, 39 (1-2), 195-204 described hydrothermal reactions over ZSM-5 on ceramic foams.
Hedlund, J. et al., The synthesis and testing of thin film ZSM-5 catalysts. Chemical Engineering Science 2004, 59 (13), 2647-2657 described hydrothermal reactions over ZSM-5 on silica and alumina supports.
He, C. et al., Synthesis and characterization of Pd/ZSM-5/MCM-48 biporous catalysts with superior activity for benzene oxidation. Applied Catalysis A: General 2010, 382 (2), 167-175 described the growth of Pd-doped ZSM-5 onto MCM-48 via a simple overgrowth method.
Öhrman, O. et al., Synthesis and evaluation of ZSM-5 films on cordierite monoliths. Applied Catalysis A: General 2004, 270 (1-2), 193-199 described a seeded hydrothermal growth method using silicalite-1 seeds supported on cordierite to form ZSM-5 films.
Yang, G. et al., Preparation, characterization and reaction performance of H-ZSM-5/cobalt/silica capsule catalysts with different sizes for direct synthesis of isoparaffins. Applied Catalysis A: General 2007, 329 (0), 99-105 described the hydrothermal synthesis of an H-ZSM-5 catalyst supported on a cobalt-silica pellet. The authors reported that the core size and morphology substantially affected the thickness and crystal structure growth of ZSM layer. The authors also reported a correlation between size and conversion efficiency: Smaller sized pellets resulted in higher conversion rates. The authors speculated that the core morphology could affect catalyst structure morphology to such a degree that reaction selectivity might be altered.
Adjaye, J. D. and Bakhshi, N. N., Production of hydrocarbons by catalytic upgrading of a fast pyrolysis bio-oil. Part I: Conversion over various catalysts. Fuel Processing Technology 1995, 45(3), 161-183 discloses the use of zeolite catalysts, including HZSM-5, in the pyrolytic production of bio-oil.
Zhang, Q.; Chang, J.; et al., Review of biomass pyrolysis oil properties and upgrading research. Energy Conversion and Management 2007, 48(1): 87-92 discloses the use of metal/metal oxide catalysts to convert smaller oxygenates to higher molecular weight compounds containing less oxygen, and simultaneously to deoxygenate phenols.
There remains an unfilled need for improved methods for making bio-oil from biomass, for improved catalysts that are useful in such transformation reactions, and for improved methods for making such catalysts.